Method for vacuum degassing and casting molten metal with electromagnetic control



ETAL

2 Sheets-Sheet l ug. 19 i970 J. s. GERO METHOD FOR VACUUM DEGASSING ANDCASTING MOLTEN M WITH ELECTROMAGNETIC CONTROL Flled May 20, 1968 Aug. L1970 1. B. GERO 3,523,785

METHOD FOR VACUUM DEGASSING AND CASTING MOLTEN METAL WITHELECTROMAGNETIC CONTROL Filed May 20, 4155168 2 Sheets-Sheet 3 Daan/NAUnited States Patent O 3,523,785 METHOD FOR VACUUM DEGASSING AND CAST-ING MOLTEN METAL WITH ELECTROMAGN ET- IC CONTROL John Bernard Gero,Manchester by the Sea, Mass., as-

signor to Gero Metallurgical Corporation, Gloucester, Mass., acorporation of Delaware Continuation-in-part of application Ser. No.498,363, Oct. 20, 1965. This application May 20, 1968, Ser. No. 730,383

Int. Cl. C21c 7/06; B22d 37/00 U.S. Cl. 75-12 9 Claims ABSTRACT OF THEDISCLOSURE Molten metal degassed in a vacuum chamber is released througha discharge conduit communicating with the atmosphere. The atmosphericpressure outside the chamber is much greater than the pressure thereinand the pressure difference tends to prevent metal from flowingout o fthe vacuum chamber. To provide continuous flow, without requiringmaintenance of a large head of metal within the chamber,electromagnetically induced forces act on the metal within the conduit,forcing it out of the chamber.

This application is a continuation-in-part of my copending applicationSer. No. 498,363, led Oct. 20, 1965, now

abandoned and relates generally to improvements in casting molten metalfor use in the steel-making industry. In one specic aspect, theinvention is concerned with the type of casting operation in whichmolten metal to be cast is rst subjected to a vacuum degassing process.

In carrying out methods of casting in which molten metal is firstsubjected to a vacuum degassing operation, problems arise in controllingthe llow of molten metal in a satisfactory manner. This is especiallythe case when the stream of molten metal is discharged from theevacuated degassing enclosure into the much higher pressure atmosphere.Heretofore, in order to discharge molten metal from a degassingenclosure while maintaining the enclosure in an evacuated condition, ithas been necessary to maintain a barometric head of molten metal(typically a column of four to iive feet in height, the exact heightdepending on the pressure differential between the inside and outside ofthe degassing enclosure), plus an additional head of 18 to 24 inches topromote ilow, within the enclosure in order to cause molten metal toiiow outwardly through the bottom discharge outlet. The reason for this,of course, is that the higher atmospheric pressure outside the enclosureexerts a force continuously seeking to displace molten metal back intothe vacuum chamber. To overcome this atmospheric-pressure-force, long,refractory-lined, heated barometric legs (such as those shown in U.S.Pats. Nos. 2,837,790, assigned to the Ford Motor Co., and 2,734,240assigned to the Allegheny Ludlum Steel Corp., and my prior Pat. No.3,146,288) have been provided between the outlet and the major portionof the degassing enclosure. Such barometric legs have provided thenecessary downward discharge force, but at considerable cost. Such legsnot only require large additional amounts of refractory and result inconsiderable heat loss but also make it impossible properly to vacuumdegass the initial few tons of the batch of molten metal passed throughthe enclosure.

Accordingly, it is a principal object of the present invention toprovide a method of controlling the flow of molten metal through thenozzle of a tundish or other vacuum casting member which eliminates theneed for long barometric legs and, thereby, results in a simple andrelatively inexpensive system which can be used for f. ICC

pouring in either air or vacuum without the use of extra pouring boxes,substantially eliminates the need of extra refractory, materiallyreduces heat loss, and permits vacuum degassing of all but a few poundsof the batch of molten metal passed through the enclosure.

The method of the present invention accomplishes these and other objectsby exerting on the metal downward electromagnetic forces to create anelectromagnetic head greater than the diierence between the vacuumpressure head produced by the pressure differential without and Withinthe enclosure and the ferrostatic head created by the molten metalWithin the enclosure. In preferred modes, this force is varied in accordwith changes in the ambient discharge pressure and head of metal withinthe enclosure, and the force is further controlled in such a manner asto maintain metal Within the nozzle to eiectively seal the enclosure tokeep it in an evacuated condition.

Other objects, features and advantages will appear from the followingdetailed description of a preferred embodiment of the invention, takentogether with the attached drawings in which:

FIG. 1 is a cross-sectional view illustrating one form of castingapparatus and indicating diagrammatically continuous formation of moltenmetal into ingot material employing the method of the invention;

FIG. 2 is a plan cross section taken on the line 2--2 of FIG. 1; and

FIG. 3 is a schematic view illustrating the method of the invention.

Considering the preferred embodiment of the invention shown in FIGS. 1and 2, numeral 2 denotes a standard continuous casting tundish havingmounted around the upper end thereof in sealed relationship a vacuumenclosure or degassing hood 4, A vacuum pump, not shown in the drawings,is connected at one side of the hood to an outlet 6 to remove gases inthe direction indicated by arrow A.

Also supported in sealed relationship with the degassing hood 4 at itsupper side is a Ivacuum cover 8 formed with an opening 10 and a sighttube 12. Attached to this cover 8 is a bottom pouring stopper controlladle 14 having an adjustable stopper element 16 for opening and closinga discharge outlet in the bottom of the ladle 14 and regulating the flowof molten metal M from the ladle through the vacuum enclosure 4 and into`the tundish 2.

At its lower side, the tundish 2 is formed with a restricted nozzleportion 18 and nozzle extension 24, defining a cylindrical passageway 19in which a column C of molten metal is confined to seal the outlet.Nozzle extension 24 is formed with an upper flange portion 24a whichmay, for example, be secured by bolts at 23 to a correspondingly flangedpart 25 of the tundish nozzle 18. At its lower side the nozzle extensionis formed with another flanged part 24b to which is secured by bolts 28a bottom member 26.

Received within the nozzle extension 24 is an annular body 30 ofelectrically insulated material and extending helically around theannular body 30 are a series of turns ofan electrical conductor 32 whichis connected to a suitable source of electrical power. As is well knownin the art, the turns 32, when electrically energized, constitute ineffect a solenoid device producing an electromagnetic eld which mayinduce a tractive or pulling force on a ferrous type body, e.g., themolten metal, extending through the turns. Other known devices capableof producing such a lield may also be used.

Metal is continuously released through passageway 19 and a nozzleopening 27 in bottom member 26 into a relatively small stream of metal Sand collects in a mold, for example, the illustrated continuousoscillating casting cylinder 17, to form a partially hardened ingot 20.The

3 oscillating casting cylinder is provided with cooling coils 22 whichregulate hardening of the molten metal in a manner well known in the artof continuous casting.

In practicing a preferred mode of the present invention, molten metalfrom ladle 14 is introduced into tundish 2 by opening stopper element 16in the bottom of the ladle. As soon as the passageway 19 at the base ofthe tundish has been sealed, by a column C of molten metal, the vacuumpump is actuated to evacuate the degassing enclosure. An operator viewsthe interior of tundish 2 through sight tube 12 and controls stopperelement 16 to maintain the molten metal within the tundish at thedesired level.

To prevent metal from freezing within passageway 19 and to provide thestability in the rate of tlow of the stream S desired for continuouscasting, it is necessary that there be a continuous, and preferablyuniform, ow of molten metal through the passageway. The forces ofgravity act on the metal within tundish 2 and create a ferrostaticpressure head which tends to move the molten metal downwardly throughthe passageway. At the same time, difference in pressure between theoutside atmosphere and the evacuated interior of tundish 2 creates avacuum pressure head which acts on the metal at the outlet of nozzleextension 24 and tends to move the molten metal upwardly. lf metal is toflow from tundish 2 through. passageway 19, it is apparent that thetotal downward force acting on the molten metal must be greater than theupward force created by the vacuum pressure head. In the past, downwardflow has been insured by providing a ve to seven foot ferrostatic head.With the present invention, a ferrostatic head of, typically, less than18 to 24 inches (measured from the bottom of passageway 19) is combinedwith downwardly-acting electromagnetic head provided by coils 32 toforce metal out of enclosure 2.

The exact magnitude of the electromagnetic head re quired varies anddepends, among other things, on the ferrostatic head within enclosure 2,the vacuum pressure head caused by the difference in pressure within andwithout enclosure 2, and the frictional and other forces required tocause flow within nozzle 18 and nozzle extension 24. AS shown in FIG. 3,the ferrostatic pressure head (PF) within enclosure 2 rises rapidly asmetal is initially poured into the tundish until the metal level reachesthe predetermined level at which it is maintained. At the time (tp) whenthe vacuum pump is rst actuated, the ferrostatic head (PF)` may besufficiently great to cause ow without requiring downwardelectromagnetic force. As the tundish is evacuated and the vacuumpressure head (PV) rises, an electromagnetic head (PE) must be inducedby coils 32 so that the total downward force (PT) is suiciently great toforce metal out of the enclosure at the desired rate. For the majorportion of the casting operation, when the vacuum pressure head (Pv) ismuch greater than the ferrostatic head (PF), the downwardelectromagnetic head (PE) is required not only to provide downward flowbut also to prevent the vacuum pressure head (Pv) from forcing the metalback into the tundish, thereby breaking the vacuum seal in passageway19. As indicated in FIG. 3, the total downward force (PT), i.e., the sum(PF4-PE) of the electromagnetic and ferrostatic heads, must at all timesbe greater than the -vacuum head (Pv).

As previously mentioned, an operator controls stopper element 16 tomaintain the molten metal within tundish at the desired level at whichthe ferrostatic head (PF) is far less than the Vacuum pressure head(Pv). The electromagnetic tractive force required for continuous anduniform ow is controlled either automatically or by a second operator,such as the operator who normally controls the speed at which thecontinuously formed ingot is withdrawn from oscillating casting cylinder17.

When the operator controlling stopper element 16 sees that the lastmetal in ladle 14 has passed into evacuated 4 review-W i I i tundish 2and thereby been completely degassed, the vacuum pump is deactivated anda valve, schematically shown as valve 34 in the side of tundish 2,opened to permit the pressure within the tundish to return toatmospheric. The second operator continues to apply such electromagnetictractive force as is required to pump all the molten metal out of thetundish.

With the above-described method of the present invention, it ispossible, for the rst time, to maintain both a high vacuum andcontinuous and uniform metal flow though the ferrostatic head Of themetal within the enclosure is always, except for a brief initial moment,far less than the vacuum pressure head. The method, in addition, reducesthe amount of un-degassed metal at the beginning of each batch fromseveral tons to a few hundred pounds, and provides for a greatlyincreased stability in the rate of flow into the ingot pouring mold orcylinder.

It will :be understood also that in the apparatus of FIGS. l and 2,various other means for producing an electromagnetic head may beutilized. For example, a plurality of separately energized wires orother electromagnet devices may be used in the nozzle extension or evenin conjunction with the oscillating sleeve 17. In addition, currentregulating means may be employed and operated in some desiredrelationship with respect to the rate of formation of an ingot or therate of solidifying of the molten metal.

Vario'us other embodiments within the scope of the appended claims willoccur to those skilled in the art.

I claim:

1. In a method of casting molten metal in which molten metal isintroduced into an enclosure body and released from said enclosure bodythrough a discharge conduit, the steps including:

sealing said body;

evacuating said body to create a vacuum pressure head caused by thedifference in pressure outside and within said enclosure body;

maintaining molten metal within said enclosure body to create aferrostatic pressure head of said molten metal;

exerting on molten metal within said discharge conduit electromagneticforces to create an electromagnetic head; and

maintaining the sum of said electromagnetic and ferrostatic headsgreater than said vacuum pressure head, said vacuum pressure head beinggreater than said ferrostatic head.

2. The method of claim 1 including the step of continuously maintainingmolten metal within said enclosure at a level such that said ferrostatichead is less than said vacuum pressure head.

3. The method of claim 1 including the step of varying saidelectromagnetic head in accordance with changes in said ferrostatichead.

4. The method of claim 1 including the step of varying saidelectromagnetic head to maintain the movement of metal from saiddischarge conduit at a substantially constant rate.

5. A method of removing molten metal from an evacuated degassingenclosure through a discharge conduit therein having its outlet endexposed to an ambient dis charge pressure higher than that pressurewithin said enclosure, Said difference in pressure creating a vacuumpressure head and molten metal within said enclosure creating aferrostatic pressure head, said vacuum pressure head being greater thansaid ferrostatic head, comprising the steps of:

exerting on molten metal within said discharge conduit electromagneticforces to create an electromagnetic head; and

maintaining the sum of said electromagnetic and ferrostatic headsgreater than said vacuum pressure head.

6. The method of claim 5 including the step of introducing molten metalinto said enclosure to continuously maintain a predetermined level ofmolten metal therein.

7. The method of claim 5 wherein molten metal is introduced into saidenclosure to seal said discharge conduit prior to evacuating saidenclosure.

8. The method 0f claim 7 including the steps of evacuating saidenclosure subsequent to sealing said discharge conduit and raising thepressure within said enclosure to a pressure substantially equal to saidambient discharge pressure prior to removing all of said molten metalfrom said enclosure.

9. The method of claim `8 wherein said ambient discharge pressure isatmospheric pressure.

References Cited UNITED STATES PATENTS 2,743,492 5/1956 Easton 164--1473,263,283 8/ 1966 Allard 164-49 3,146,288 8/1964 Gero 266-34 3,210,81110/ 1965 Schneider 266-38 X 3,268,958 8/ 1966 Sickbert 164--49 ROBERT D.BALDWIN, Primary Examiner U.S. C1. X.R.

